1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system and, in particular, to a magnetic imaging system which can obtain separated images of two substances having magnetic resonance frequencies somewhat different in their chemical shift with high speed.
2. Description of the Related Art
As well known in the art, a magnetic resonance (MR) imaging is a technique of imaging the chemical and physical information of molecules, utilizing the phenomenon that when a group of nuclear spins of atoms having their inherent magnetic moment is placed in a homogeneous static magnetic field of strength HO it is possible to resonantly absorb the energy of a high frequency magnetic field rotated at a specific angular speed .omega.=.gamma.HO within a plane perpendicular to the direction of the static field where Y denotes a gyromagnetic ratio.
As a method for imaging a spatial distribution of specific nuclei, such as the nuclei of hydrogen in water and fat, of an object through the utilization of the magnetic resonance imaging, use is made of a projection reconstruction method by Lauterbur, Fourier transform method by Kumar, Welti and Ernst, spin warp method (modified Fourier transform method, and so on).
The magnetic resonance imaging system for obtaining an image by virtue of a magnetic resonance imaging takes a longer time in the collection of data than the other medical image diagnosing apparatus, e.g., an ultrasonic diagnosing apparatus or X-ray CT (Computer tomography) apparatus. Therefore, an artifact is produced by the movement of the object, e.g., a patient, such as the respiration movement and it is thus difficult to obtain an image of a high-speed moving section or site, such as the heart and circulatory system. Furthermore, the patient experiences great pain during the imaging period due to longer imaging time involved.
As a method for obtaining an image at high speed by a magnetic resonance imaging technique, ultra high-speed imaging methods have been proposed, such as an echo planar method by Mansfield and ultra high-speed Fourier transform method by Hutchison.
In the echo planar method, the magnetic resonance data is collected in accordance with a pulse sequence as shown in FIG. 1.
(1) The magnetization of the slice is selectively excited by applying a 90.degree. high frequency selective excitation pulse as a high frequency magnetic field RF, while applying a slicing gradient magnetic field Gs for the slice.
(2) a 180.degree. high frequency pulse is applied.
(3) A read-out gradient field Gr is applied by a plurality of high-speed switching operations in a direction parallel to the slice plane, while a phase-encoding gradient field Ge is statically applied in a direction parallel to the slice plane and at right angles to the read-out gradient field Gr.
The ultra high-speed Fourier method also referred to as a multiple echo Fourier method is different from the echo planar method of FIG. 1 in that, as shown in FIG. 2, the phase encode gradient field Ge is applied in a pulse-like fashion for each inversion of the read-out gradient field. In other respects, both are similar to each other.
According to these methods, all data necessary for the imaging of the slice can be collected within a time period in which the magnetization of the slice excited by the single 90.degree. high frequency pulse is relaxed by the relaxation of the transverse magnetization, so that it is possible to obtain a high-speed imaging.
However, these ultra high-speed imaging methods pose the problems as will be set forth below.
The excited magnetization in the slice of the object produces a phase dispersion resulting from the spatial inhomogeneities of the static field and hence an apparent transverse relaxation occurs at a time T2* which is shorter than that transverse relaxation time T2. Even if the collection of the magnetic resonance data is to be achieved immediately after the excitation of the magnetization, it is difficult to start data collection at the onset of excitation in terms of the system's characteristics. Hence, there is an unavoidable delay at the start of data collection. A correct image cannot be reconstructed due to the influence of the phase inversion at the time T2*. In the extra high-speed imaging pulse sequence as shown in FIGS. 1 and 2, magnetization is excited by a 90.degree. selective excitation pulse and 180.degree. high frequency pulse is applied to allow the phase of the magnetization which is spatially phase dispersed to be correctly rearranged to collect image data.
In order to obtain a correct magnetization phase this method takes double a time from the excitation of magnetization until the application of a 180.degree. high frequence pulse. The magnetic resonance data collection time and the time required from the 180.degree. pulse application are added to the time (i.e. imaging time) from the excitation of the magnetization until the collection of the magnetic resonance data is completed. That total time offers a bar to the implementation of high-speed imaging.
In the aforementioned ehco-planar and ultra high-speed Fourier transform methods, the intensity of the phase encode gradient field is very small, thus being liable to produce a phase encode error resulting from the spatial or time inhomogeneities of a static field. Such a phase encode error causes a deformation or blurring of an image reconstructed.
A magnetic resonance signal, even if coming from the nuclei of specific atoms in the sample, produces a frequency difference called a "chemical shift" due to a difference in the chemical circumference. The Dixon method is known as the method for obtaining a water and fat separated image through the utilization of the chemical shift. In accordance with the Dixon method, two imaging operations are required in obtaining a water/fat-separated image. In this case, the Dixon method uses the conventional imaging technique and takes a longer time for the respective imaging than the aforementioned ultra high-speed imaging method. Hence it takes longer time to obtain a water/fat-separated image.
FIGS. 3A and 3B show a pulse sequence for obtaining a water/fat-separated image by the Dixon method.
As shown in FIG. 3A, a gradient field Gs for the slice and 90.degree. selective excitation pulse RF.sub.1 are applied to the object and, upon applying a 180.degree. pulse RF.sub.2 following a time TE/2, the nuclear spin dispersed is brought into an in-phase relation after TE/2, producing an echo. At this time, magnetic resonance signals of the nuclei of hydrogen in water and fat have the same phase. Under these situations, the read-out gradient field Gr is applied, allowing the collection of data for one echo. By repeating these operations a plurality of times while varying little by little the phase encode gradient field Ge, it is possible to collect whole magnetic resonance data necessary for the reconstruction of an image. If the image is reconstructed by the magnetic resonance data thus obtained, then it is possible to get an image corresponding to a sum of the water and fat images, noting that this image contains a chemical shift, that is, a positional displacement corresponding to a difference between the frequence of the water and that of the fat.
As shown in FIG. 3B, the application timing of the 180.degree. pulse RF.sub.2 is changed a time TE'/2 from a 90.degree. pulse RF.sub.1 and magnetic resonance data is collected a time .DELTA.T after the occurrence of an echo following that time .DELTA.T. At this time, the time .DELTA.T is so set that a phase difference between the magnetic resonance signals of the nuclei of hydrogen in the water and fat is just 180.degree.. The image reconstructed based on the magnetic resonance data thus obtained corresponds to an image which is a difference of the image of the water and that of the fat. It is thus possible to separate the image of the water and that of the fat by computation from those two reconstructed images.
In the Dixon method, two imaging steps are needed to obtain a water/fat-separated image. If, however, the time .DELTA.T in the sequence of FIG. 3B is set to a predetermined value, that is, the time at which a phase difference between both the water and the fat signal is .pi./2 or -.pi./2, then the water and fat image information can be obtained by one imaging operation in a manner separated into a real and an imaginary part of the image data, respectively. The method for obtaining the water/fat-separated image through the utilization of the above is referred to as a modified Dixon method. Even the modified Dixon method employs conventional imaging method which is longer in the imaging process than the ultra high-speed imaging method and hence it takes longer time to obtain a water/fat-separated image.